The competition watchdog will be criticised next week over the time it is taking to complete its most important investigations when it confirms that a banking probe is being extended by several months.

Sky News understands that the Competition and Markets Authority (CMA) has in recent weeks been leaning towards seeking the maximum six-month delay to the publication of a report on the banking sector.

The watchdog's preliminary findings, published last year, were attacked for being insufficient to break the stranglehold of the UK's biggest high street lenders on personal current accounts and small business banking.

A CMA spokesman refused to comment on the length of the extension it would seek.

The regulator told stakeholders on Friday that it would publish the details of its plans, including a working paper on a proposal for a price comparison website for SMEs, next Monday.

Sources said MPs were "furious" at the delay, adding that CMA officials were likely to be summoned to explain them.

Challenger banks have lined up to castigate the CMA since its initial proposals were outlined in October.

In a letter obtained the following month by Sky News, Paul Lynam, the chief executive of Secure Trust Bank, said he was "astonished" that the CMA had described its proposals as radical.

"The possible remedies proposed thus far can in no way be considered far-reaching or thorough, not least as there are no meaningful proposals to address the huge disadvantages suffered by small banks in terms of disproportionate capital, expensive funding, access to payments infrastructure and excessive regulation," he wrote.

Mr Lynam argued that far from improving competition, some of the CMA's ideas would instead favour the so-called 'Too Big To Fail' lenders, meaning the UK's largest high street banks which already dominate the personal current account and SME banking markets.

"Overall the remedies need to be fundamentally rethought and targeted to drive wholesale changes across the board rather than be almost exclusively limited to measures which, at best, will only result in marginal incremental changes in current account switching," he added.

The CMA's provisional findings suggested that bank current account customers could save an average of £70 annually by switching providers, placing the onus on users of banking services to be more active in pursuing better deals.

It added that Britain's free-if-in-credit current accounts model should not be abolished, because there "is no evidence that it distorts competition".